1. Field of the Disclosure
This disclosure relates generally to estimating characteristics of fluids from the measurements of thermal properties of the downhole fluids.
2. Description of the Related Art
Wellbores or boreholes for producing hydrocarbons (such as oil and gas) are drilled using a drill string that includes a tubing made up of jointed tubulars or a continuous coiled tubing and a drilling assembly, also referred to as the bottom hole assembly (BHA), attached to the bottom end of the drill string. The BHA typically includes a number of sensors and tools that are used for evaluating properties of the formation and for drilling directional boreholes. A drill bit attached to the BHA bottom is rotated with a drilling motor in the BHA and/or by rotating the drill string to drill the wellbores. To drill a wellbore, drilling fluid, also referred to as the “mud,” is supplied under pressure to the drill string, which mud discharges at the bottom of the drill bit and circulates back to the surface via an annulus between the drill string and the wellbore inside.
A majority of the wellbores are drilled under overbalanced conditions, wherein the pressure on the formation surrounding the wellbore due to the weight of the mud column is greater than the natural or connate pressure of the formation. The drilling mud invades the formation to a certain depth and contaminates the connate fluid (fluid present in the formation under natural conditions). It is desirable to estimate or determine the characteristics or properties of interest of the fluid in the formation during drilling of the wellbore. These estimates can then be used to control drilling of the wellbore and to estimate the presence of hydrocarbons. Formation fluid samples also may be taken during drilling of a wellbore and/or after a well has been drilled. To obtain a relatively clean (substantially free of mud filtrate) fluid sample, the formation fluid is typically pumped into the wellbore until clean or uncontaminated formation fluid starts to flow out of the formation. Invasion is less during drilling of a wellbore compared to the invasions after a few hours after the wellbore has been drilled under overbalanced conditions. It is therefore desirable to determine when the formation fluid being withdrawn is clean so that a formation fluid samples may be taken.
The present disclosure provides a downhole tool and method for estimating certain characteristics of downhole fluids, including estimating the contamination of the fluid.